The fail of hard disk
When the Spindle Motor does not activate.
When the Partition Table has been deleted.
Missing Operation System
Invalid Media Type
Deleted Data
Formatted
Overwriting
Low-Level Format
  Formatted
 
1.

When not saved after formatting, you usually can recover your files by the command UNFORMAT.

   
2.

If you have used you computer for a long time after formatting, and saved another program or overwritted, you replaced the space where the original data was, so in most of these cases it is difficult to recover your data.

   
3. Sometimes you can confuse the C: to the D: and Delete or Format your data.